Powertrain, generic · Hybrid propulsion

P0AC4 Hybrid/EV Powertrain Control Module Requested MIL Illumination

Check engine lightUsually off
EmissionsRelevant
Drive cycleNot required
Limp modeNo

In many hybrids the hybrid powertrain controller has its own emission-related diagnostics but does not drive the malfunction indicator lamp itself. When it detects such a fault it sends a MIL request to the engine controller, which then stores P0AC4 as a pointer. GM documents this in its 2011, 2013, 2016 and 2019 hybrid tables: the engine controller watches the request message continuously once 3 seconds have passed after power-up and logs P0AC4 after one trip, listed as Type A but marked No MIL because the lamp is owed to the original fault. The real cause is therefore stored in the hybrid controller and must be read there. P0A7B is the equivalent request from the battery energy control module.

Likely causes

Emission-related fault stored in the hybrid controller
Request still present because codes were cleared only in the engine controller

Symptoms

  • Check engine light on (caused by the underlying hybrid fault)
  • Further codes present only in the hybrid controller

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–200
Time0.5–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

hybrid powertrain cmmodule communication bus

Related codes

P0A7BP0AB9

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com.